Originally Posted by SoundsGood
About 13' or so.
Wonderful. This places the listeners at roughly 1/3rd the room length, which will help make the frequency response smoother.
See the attached diagram of a typical 7.1-speaker layout. Notice the side speakers are in line with the main seating and the rear speakers are spread around 60 degrees apart behind the listeners.
If you want something similar to the diagram, then mount your side speakers directly to the sides of your seating area and mount your rear speakers about 14 feet back (still on the side walls). You'll end up with the same angles as on the diagram. Remember to aim/point the rear speakers towards the listening area. I would mount all 4 surrounds at least a couple of feet above ear level.
When a car exits screen left, the sound will disappear to your left. When a plane flies overhead, the sound will disappear behind you. Difficult, if not impossible, to get the same results with only one pair of surrounds.